Remove stdcall support

When I worked on an initial GHC patch to enable cross compiling to arm64 windows (see #24603 (closed)), I noticed a common code pattern in the GHC tree as well as some boot libs: using CPP to check i386/x86_64 host and use stdcall/ccall calling convention accordingly.

Given we have dropped support for i386 windows quite a while ago, I believe all stdcall related code paths have become essentially unused and are undesired maintenance burden when adding arm64 windows support. Shall we remove it completely?